It needs to be withing the loop for the comments, and you want to specify it so that it outputs into a class attribute. Something like this:
<div class="<?php author_highlight(); ?>">
This is a comment.
</div>

FROM MY COMMENTS.PHP FILE
<?php // Do not delete these lines
if ('comments.php' == basename($_SERVER['SCRIPT_FILENAME']))
die ('Please do not load this page directly. Thanks!');
if (!empty($post->post_password)) { // if there's a password
if ($_COOKIE['wp-postpass_' . COOKIEHASH] != $post->post_password) { // and it doesn't match the cookie
?>